The aim with the invention is to provide an electrochemical composition which is printable, and to provide a printable electrochemical device by printing an electrolyte on top of a first electrode, and to print a second electrode on top of the electrolyte. Suitable electrolytes for such an application shall fulfill some requirements, they shall be printable and they shall be over-printable. These properties are in addition to their basic function on having the ability to function as an electrolyte in the device.
It is desirable that the device can be printed on a flexible substrate, for example a porous or fibrous substrate such as paper, and that it can function in a range of environmental conditions without the need for encapsulation. The condition that the device should be printable on fibrous and porous substrates challenge the mobility of the electrolyte at the same time as a minimum mobility is essential for the function of the electrolyte.
The most common group of printable electrolytes consists of either an ionic liquid in a binder system, such as in WO 2008062149 A1, or a salt dissolved in a solvent in a binder system. However, these systems comprise ions that can migrate from the place of print deposition, especially if the substrate is porous, leading to ion depletion in the printed area and contamination of other components. On the other hand, a certain degree of mobility is required for the function of the electrolyte. It is desirable that the electrolyte salt can migrate at least to some extent through the film thickness of the electrode layers between which it is sandwiched.
It has been found that an electrolyte based on polycations provide an acceptable degree of mobility in printed structures, so that ion transport between electrolyte and electrode is sufficient, while migration of the polycations on fibrous substrates is acceptably low. It has also been found that printing polyelectrolytes with screen printing can be problematic due to the amphifilic nature of many polyelectrolytes. As amphiphiles, they can stabilize foams and bubbles. Further, the high molecular weight of the polymer enables formation of bubbles and filaments. In screen printing, where fluid ink meets air and passing a fine mesh, bubble formation is well accommodated. In the ink ink-split moment, when the web is lifted from the substrate, conditions for filament formation are ideal. Filaments and bubbles can lead to poorly defined prints and pin-holes.
It is further desirable that the composition forming the electrolyte has a good printability on a first electrode, as well as the substrate surrounding it, so that a pin-hole free film can be obtained by printing. Further, the so print deposited film should be curable to a film stable enough to be overprintable by a composition forming a second electrode.
Thus, there is a need to destabilize or suppress formation of bubbles or filaments during screen printing of a printable composition based on a quaternized polycation in a cross-linkable binder system.
The most common way to reduce foam and improve leveling in inks involves adding surfactants and silicon based compounds. There are reasons to avoid typical surfactants in an electrochemical cell. The surfactants can form thin insulating layers reducing the ionic connectivity in the cell. It is also known that addition of hydrophobic particles can limit foaming in oil based inks as well as in water-based inks and that hydrophilic particles can stabilize foaming.
Printable electrolytes are previously known. US2006/0199059A1 discloses polyelectrolyte composition which is printable, curable, and contains rigid particles. Herein the curable polymer is conductive as well as the rigid particles.
WO2010/018370 describes an anion exchange polymer obtained by quaternization of tertiary amines by poly(vinylbenzyl chloride). The polymer is deposited and cured on electrodes by printing, also together with solid particles. The quaternised amine is polymerized when curing the composition. The electrodes are to be used in an electrochemical device like a fuel cell.
In WO98/42037 an electrolyte system comprising a blend of polymeric support structures together with electrolyte active species dispersed in the support structure is described. The electrolyte active species are selected from mineral acids or metal hydroxides.
In U.S. Pat. No. 5,284,894 is a low-foaming latex described. Herein is a polyelectrolyte added to a fortified latex to reduce foaming.
A composition comprising a polycationic polyelectrolyte is disclosed in EP 1664167 B1. The composition is included in a process for preparing a humidity sensor by inkjet printing. The disclosed polyelectrolytes have a crosslinkable functional group.
US 2003/0131756 A1 describes an ink composition comprising precipitated complexes of polyquaternary amines and anionic dyes. The composition is suitable for inkjet printing processes.
EP 2169746 describes a catalytic ink in form of a dispersion, and which also comprises polyelectrolytes.
In U.S. Pat. No. 4,654,279 is a solid polymeric electrolyte including two phase electrolyte ink described. One of the phases shows high ionic conductivity and the other phase forms a mechanically strong network and has a slight ionic conductivity. The ionic conducting phase comprises complexed metal salt.
Printable ion gels based on low molecular ionic liquids are disclosed in Cho et al., Nature Materials, 900, 2008, and Ollinger et al, Appl. Surf. Sci., 252, 8212, 2006. These ion gels are suitable for flat substrates.
There are no descriptions of printing processes or printability, or overprintability of compositions forming electrolytes in the aforementioned publications.
Therefore, there is a need for an electrolyte composition comprising polymeric polyelectrolytes which are printable. Especially, the electrolyte composition shall be suitable to be printed upon an electrode material, to form a vertical electrochemical cell. The electrolyte may be suitable in different printing techniques, such as screen printing, in flat-bed printer or rotary printing.
The electrolyte composition of the invention will upon curing form a layer of electrolyte that cover the area of the underlying electrode, the first electrode, substantially without any interruptions in its structure. The printable electrolyte shall also provide good adhesion to the underlying layers as well as being printable, also when aqueous-based composition being printed onto the electrolyte.